1. Field of the Invention
This invention relates generally to blown film extrusion lines, and specifically to operations of the inlet and outlet blowers of blown file systems.
2. Description of the Prior Art
Blown film extrusion lines are used to manufacture plastic bags and plastic sheets. A molten tube of plastic is extruded from an annular die, and then stretched and expanded to a larger diameter and a reduced radial thickness by the action of overhead nip rollers and internal air pressure. Typically, air is entrained by one or more blowers to provide a cooling medium which absorbs heat from the molten material and speeds up the change in state from a molten material back to a solid material. Additionally, blowers are used to provide air pressure which is utilized to control the size and thickness of the film tube. One type of blown film extrusion line utilizes an air flow on the exterior surface of the film tube in order to absorb heat. A different, and more modern, type of blown film extrusion line utilizes both an external flow of cooling air and an internal flow of cooling air in order to cool and size the film tube.
As stated above, blowers are utilized to provide air to the interior of the film tube. Typically, a supply blower is provided in order to supply air to the interior of the film tube, and an exhaust blower is provided in order to exhaust air from the interior of the film tube. Typically, the supply blower and exhaust blower are under electrical control during production operations. However, during startup of the extrusion process, in the prior art, a great deal of human intervention is required in order to establish the bubble. Typically, a human operator will first control the supply blower until the extruded film tube is closed at its upper end by engagement with the overhead nip rollers. Then, the exhaust blowers utilized to remove air in order to prevent expansion and eventual breaking of the extruded film tube. A balance between the supply blower and the exhaust blower must be obtained in order to allow for continuous production of the extruded film tube. The startup of an extruded film is a relatively difficult operation to perform, and generally requires a relatively highly-skilled employee to oversee or perform the startup operations.